It comes after two men were arrested under the Official Secrets Act amid reports a parliamentary researcher spied for China.

The leaders met at the G20 in Delhi. It is understood the meeting between Mr Sunak and Li Qiang, who is attending the G20 in place of Chinese president Xi Jinping, was unplanned and happened at the summit's final session on Sunday morning.

The researcher has links to several senior Tory MPs and has been arrested on suspicion of spying for China, according to The Sunday Times.

The researcher, who is in his 20s, is understood to have had links to security minister Tom Tugendhat, foreign affairs committee chairwoman Alicia Kearns and other senior Tory MPs.

Mr Sunak has been clear China is an "epoch-defining" challenge, Justice Secretary Alex Chalk told Sky News' Trevor Phillips on Sunday.

He defended security in parliament, saying a "rigorous approach is taken" in terms of who is given a parliamentary pass.

Asked if there are weaknesses in the security system that need addressing, he said: "I don't think you should rule anything out".

Officers from the Metropolitan Police's Counter Terrorism Command, which oversees espionage-related offences, are investigating.

Mr Chalk said the investigation must "play through" and police and security services are taking this "seriously".

"Whatever lessons need to be learned by the parliamentary authorities I'm sure will be learned," he said.

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Asked about the view of some in parliament that China is a threat to the UK, Mr Chalk said "you can't wish China away", noting it is second largest global economy and key to tackling the challenge of climate change.

"We have to engage, but we do so with our eyes open," he said.

One of the men, in his 30s, was arrested in Oxfordshire on 13 March, while the other, in his 20s, was arrested in Edinburgh, Scotland Yard said.

"Searches were also carried out at both the residential properties, as well as at a third address in east London," a statement from the force said.

The MPs he is linked to are privy to classified or highly sensitive information.

Both men were held at a south London police station until being bailed until early October.

Mr Tugendhat is said not to have had any contact with the researcher since before he became security minister in September last year.

Ms Kearns declined to comment, adding: "While I recognise the public interest, we all have a duty to ensure any work of the authorities is not jeopardised."

The Inter-Parliamentary Alliance on China pressure group said it was "appalled at reports of the infiltration of the UK Parliament by someone allegedly acting on behalf of the People's Republic of China".

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